Pioneer efforts in spectrophotometric analysis of dairy products are reported in Goulden, "Infra-red Spectroscopy of Dairy Products," J. Sci. Food Agric., Sept. 7, 1956, pp. 609-613. In this paper, Dr. Goulden reported milkfat absorption peaks or "shoulders" at wave numbers of about 2900 to 2850 cm.sup.-1, 1740 cm.sup.-1 and 1465 cm.sup.-1, among others, corresponding to wavelengths of 3.48 microns, 5.73 microns and 6.83 microns respectively. Absorption at the 6.83 micron wavelength was shown to be relatively weak as compared with those at the 3.48 and 5.73 micron bands. Dr. Goulden also reported in U.S. Pat. No. 3,161,768 that, due to the so-called Christenson light-scattering effect of milkfat globules, it is necessary that the fat measurement wavelength be at least three times the average globule diameter to obtain accurate fat assessments employing this technique. Given the state of the homogenization art at that time, and in view of absorption characteristics of water, the major component of milk, the wavelength of 5.73 microns, characteristic of ester linkages in the triglyceride portion of the fat molecule, was selected as "most convenient."
Thus, Goulden U.S. Pat. No. 3,161,768 and "The Infra Red Milk Analyzer," J. Soc. Dairy Tech, 17, 1 (1964) p. 28-33 disclose an analyzer for measuring concentrations of fat, protein, lactose and non-fat solids in dairy products. This analyzer, marketed under the trademark IRMA, measured absorption of infrared energy at selected wavelengths characteristic of each constituent, and cross-corrected among the raw fat, protein and lactose readings for effects due to the other constituents. Subsequent advances in the art of small homogenizers suitable for use in devices of this type yielded milkfat particle sizes of 1.2 microns or less. Such a homogenizer was contained, for example, in the Milko-Scan unit described as prior art in Nexo et al U.S. Pat. No. 4,247,773. According to the teaching of Dr. Goulden that particle size must be no more than about 1/3 of the milkfat measurement wavelength, it was then feasible to measure milkfat accurately at the 3.48 micron wave band disclosed by Dr. Goulden characteristic of stretching of carbon-hydrogen bonds in the fatty acid chain. Thus, the Nexo et al patent discloses a milkfat measurement technique wherein measurements are performed at the 3.48 micron wavelength rather than the 5.73 micron wavelength of the IRMA and prior Milko-Scan units.
Each of the 3.48 and 5.73 micron wavelengths possesses advantages and disadvantages relative to measurement of milkfat concentration. The 5.73 micron wavelength is responsive to triglycerides, specifically ester linkages, in the milkfat molecules, but substantially independent of variations in fatty acid chain length. Stated differently, 5.73 micron wavelength measurement assumes uniform mean molecular weight. However, genetic variations among cow breeds and the practice of employing differing feedstuffs have caused significant variation in mean molecular weight of milkfat among differing herds. The 3.48 micron wavelength, on the other hand, is responsive to stretching of saturated carbon-hydrogen bonds in the milkfat molecule, and thus is more closely, but not precisely, responsive to variations in fatty acid chain length. However, measurement at the 3.48 micron wavelength is subject to substantial variation due to natural variations in degree of saturation and the number of hydroxyl and methyl groups per molecule. In addition, 3.48 micron wavelength measurements vary significantly with protein and lactose concentrations, rendering cross-correction more difficult. Thus, the 3.48 micron wavelength technique is of little value as applied to skim milk products, where lactose, protein and water concentrations greatly overshadow fat concentration, and in synthetic dairy products prepared with polyunsaturated vegetable oils.
A detailed scientific study of the causes of inaccuracy of the 3.48 and 5.73 micron measurement techniques by Prof. D. Biggs led to the discovery that, while both measurements are sensitive to changes in molecular weight, the variations are of opposite sign. This observation of scientific fact led to the realization that both the 3.48 and 5.73 micron measurements are based upon absorbances which represent only a portion of the whole milkfat molecule, and that the magnitude of the errors of each are proportioned to the variation in weight of these portions or weight fractions of the molecule as a function of total weight of the molecule. Thus, Biggs et al U.S. Pat. No. 4,447,725 discloses a technique for measuring milkfat as a conjoint function of the 3.48 and 5.73 measurement readings. This technique is substantially independent of molecular weight variations and, as implemented in the apparatus disclosed in Shields U.S. Pat. No. 4,310,763 and 4,418,809 and marketed by applicant under the trademark MULTISPEC, has enjoyed substantial commercial acceptance and success in the dairy industry where compensation to milk producers is based in substantial part by law on fat concentration.
However, other problems inherent in the 3.48 and 5.73 micron measurement techniques have remained unresolved. For example, lipolysis of the triglycerides, which increases with age and poor storage, among other factors, causes deesterification of the ester linkages in the carbonyl ester bonds, and thus decreases accuracy of the 5.73 micron measurement. Accuracy of the 3.48 micron reading likewise declines, and the two inaccuracies are not of opposite sign as with molecular weight variations. Thus, even the conjoint measurement technique of Biggs et al does not accommodate inaccuracies due to lipolysis. Another problem lies in control of homogenization. Although reduction of milkfat particles to less than 1.2 microns is well within conventional technology, maintenance of this level presents difficulties as the homogenizer ages and/or is not properly maintained or operated. It is desirable to decrease such dependence of measurement accuracy upon homogenization. Another problem, which applies particularly to high-volume automated testing laboratories, lies in the fact that the Biggs et al technique requires two fat measurements, increasing the sample through-put time by 1/3 for measurement of fat, protein and lactose.
Concentration of fat in milk produced by typical cow breeds averages about 3.7 weight percent (w/o). In some cow breeds, concentration can range up to 8 to 9 w/o. For Indian buffalo, milkfat concentrations up to 12 w/o are typical. The extinction or absorption coefficients at the 3.48 and 5.73 micron wavelengths are such that measurements may be performed accurately and with high resolution on dairy products having milkfat concentration up to 15 w/o employing the standard MULTIPSEC thirty-seven micron-thick sample cell. However, such measurements cannot be readily performed on high-fat dairy products, such as ice cream and cream, because the extinction coefficients at the 3.48 and 5.73 micron wavelength are such that energy is substantially completely absorbed at 15 w/o, and therefore yields little resolution above this concentration. Use of a thinner sample would reduce absolute absorption and increase measurement range, but would also lead to clogging of the cell. Thus, it has become standard practice for persons wishing to measure milkfat concentrations in products above 15 w/o fat to dilute the product at predetermined ratio to obtain samples for test. Such dilution is time consuming and subject to inaccuracy.